Much attention has been given to methods of improving the efficiency of distillation systems, especially those used in water desalination, with a view to reducing the costs of production of purified water.
It is usual practice in distillation to condense the distillate in a condenser section of the evaporation chamber. Vapor enters the condenser section, is cooled and condensed by a cooling water coil, and gives up heat including latent heat of condensation. Cooling water enters this coil from a separate circuit, takes up the "recovered heat", and emerges at a higher temperature. The "recovered heat" is often utilized to heat feed liquid entering the evaporation chamber by using the feed as coolant in the cooling water coil.
In order to improve efficiency, it is common to use a mechanical vapor compression system in which vapor is withdrawn from the evaporation chamber and condensed by means of a centrifugal compressor. The compressor is typically the most expensive component of the plant, requiring vapor seals, bearings, oil seals and rotating parts. It is also the most easily damaged component.
There are many advantages in operating stills at sub-atmospheric pressures. Such systems operate at low temperatures for example in the range 30.degree. to 50.degree. C. At these temperatures, problems associated with the formation of scale are greatly reduced and plastics construction materials may be used. However, the difficulties in using vapor compression in sub-atmospheric systems are greater in comparison with stills operating at or above atmospheric pressure, since the compressor must extract vapor at a lower pressure.
An object of the present invention is to provide an efficient apparatus for distillation which is operable at sub-atmospheric pressures and which alleviates at least some of the disadvantages of the prior art.